1. Field of the Invention
The invention relates generally to offshore structures to facilitate offshore oil and gas drilling and production operations. More particularly, the invention relates to depth-adjustable offshore towers that are releasably secured to the sea floor and configured to pitch in response to environmental loads.
2. Background of the Technology
Various types of offshore structures may be employed to drill subsea wells and/or produce hydrocarbons (e.g., oil and gas) from subsea wells. Usually, the type of offshore structure selected for a particular application will depend on the depth of water at the well location. For instance, in water depths less than about 250 feet, conventional jackup platforms are commonly employed; in water depths between about 250 feet and 450 feet, specially designed “high spec” jackup platforms are commonly employed; in water depths less than about 600 feet, fixed platforms and compliant towers are commonly employed; and in water depths greater than about 600 feet, floating systems such as semi-submersible platforms and spar platforms are commonly employed.
Jackup platforms can be moved between different wells and fields, and are height adjustable. However, conventional jackup platforms are generally limited to water depths less than about 250 feet, and high spec jackup platforms are generally limited to water depths less than about 450 feet. Although conventional jackup platforms have low day rates, and thus, provide a low cost option in shallow waters, high spec jackup platforms have relatively high day rates and may be cost prohibitive. In addition, deployment and installation of jackup platforms, typically requiring both a launch barge and a derrick barge, can be challenging, especially in deeper waters. Jackup platforms may also be less desirable for use in earthquake zones since rigid bottom-founded jackup platforms exhibit very little compliance.
Fixed platforms include a concrete and/or steel jacket anchored directly to the sea floor, and a deck positioned above the sea surface and mounted to the upper end of the jacket. Fabrication and installation of a fixed platform requires a particular infrastructure and skilled labor. For example, launch barges are needed to transport the components of the jacket and the deck to the offshore installation site, derrick barges are needed to position and lift the upper portion of the jacket, and derrick barges are needed to lift and position the deck atop the jacket. In addition, installation of a fixed platform often requires the installation of piles that are driven into the seabed to anchor the jacket thereto. In deeper applications, additional skirt piles must also be driven into the seabed. In select geographic locations such as the Gulf of Mexico, fixed jacket platforms are fabricated, deployed, and installed on a regular basis. Accordingly, such regions typically have the experience, infrastructure, and skilled labor to enable fixed jacket platforms to provide a viable, competitive option for offshore drilling and/or production. In other regions, having little to no experience with fixed jacket platforms, the facilities, equipment, infrastructure, and labor may be insufficient to efficiently construct, deploy, and install a fixed jacket platform. Moreover, even in some regions, such as Brazil and Peru, that have some experience fabricating and installing fixed jacket platforms, the range of applications for fixed jacket platforms anticipated in the next few years may exceed present capabilities.
Fixed jacket platform are typically designed to have a natural period that is less than any appreciable, wave energy anticipated at the offshore installation site. This is relatively easy to accomplish in shallow waters. However, as water depths increase, the inherent compliance, and hence natural period, of the jacket increases. To reduce the natural period of the jacket below the anticipated wave energy as water depth increases, the jacket is stiffened by increasing the size and strength of the jacket legs and pilings. Such changes may further increase the infrastructure and labor requirements for fabrication and installation of the jacket. Similar to jackup platforms, since fixed platforms are rigid bottom-founded structures, they tend to be less desirable for use in earthquake zones.
Floating systems can be used in deep water and are suitable for use in earthquake zones since they are not rigidly connected to the sea floor. However, floating structures are relatively expensive and difficult to move between different locations since they are designed to be moored (via multiple mooring lines) at a specific location for an extended period of time. In addition, the lower ends of the mooring lines are typically anchored to the sea floor with relatively large piles driven into the sea bed. Such piles are difficult to handle, transport, and install at substantial water depths.
Accordingly, there remains a need in the art for offshore drilling and/or production bottom-founded structures anchored to the sea floor that are easily installed (e.g., lower infrastructure and specialized labor requirements) and moved between different offshore locations. Such offshore productions systems would be particularly well-received if they were economical, suitable for use in earthquake zones, and could be employed in different water depths.